Numerical analysis of an upstream tailings dam

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1 NGM 2016 Reykjavik Proceedings of the 17 th Nordic Geotechnical Meeting Challenges in Nordic Geotechnic 25 th 28 th of May Numerical analysis of an upstream tailings dam L. Hassellund, R. Knutsson, H. Mattsson & S. Knutsson Luleå University of Technology, Sweden, ABSTRACT This paper presents a case study of how the finite element method can be utilized to analyze stability of upstream tailings dams. Upstream tailings dams are usually raised gradually and the increased load normally influences the stability in an unfavorable way; the load generates excess pore water pressures and reduced stability. In this study, an upstream tailings dam in Northern Sweden was numerically simulated with the finite element software PLAXIS 2D in order to assess the stability of the dam. Upstream tailings dams are sensitive to high raising rates since initiated excess pore water pressures might not have time to dissipate. Stability analysis of a tailings dam is an application that is very suitable to carry out using finite element software; once a finite element model of the complex geometry of a dam has been established, it is easy to stepwise add new soil volumes, associated with each new raising, to the model. In this case study, it was found that strengthening actions were needed in order to maintain a stable structure. Rockfill berms were gradually added on the downstream slope of the model to obtain a factor of safety above a recommended value. The volumes of rockfill needed for the berms were minimized by numerical optimization to reduce costs. The stability between the years 2024 and 2034 was analyzed; with an annual deposition cycle. The performed numerical study resulted in a future plan for placement of rockfill berms to establish sufficient stability of the tailings dam. It was found that the volume of rockfill in the berms needed, varied during the years studied. Numerical modeling, as presented in this paper, is a useful tool for the dam owner to plan and design for future raisings of a tailings dam. Keywords: Numerical simulation, tailings dam, stability, excess pore water pressure 1 INTRODUCTION Waste material from mining facilities has to be taken care of in economic, environmental and safety aspects. This waste material is usually deposited in large impoundments surrounded by embankment dams i.e. tailings dams. The tailings dams have to be raised continuously to maintain storage for new generated tailings material. Tailings dams can be raised mainly by three different methods; upstream construction, downstream construction or centerline construction (Vick, 1990). The aim of this paper is to show how the finite element method, in this case with the software PLAXIS 2D (Brinkgreve et al., 2014), can be utilized as a tool for dam safety analysis. A tailings dam mainly built by the upstream method is investigated between the years 2024 and 2034 to assess its future stability. The tailings dam is raised annually and its stability is ensured by adding rockfill berms on the downstream slope of the dam. The locations of the berms are optimized with the aim to minimize the total volume of rockfill. If sufficient stability of the studied dam in the existing impoundment can be achieved in the future, the owner does not need to find other deposition options for the generated tailings. The tailings dam investigated in this paper is located in Aitik in Northern Sweden and has previously been examined by Ormann et al. (2013), Ormann et al. (2011) and Knutsson et al. (2015); where numerical simulations were utilized. Both in Ormann et al. (2013) and Ormann et al. (2011) the time period studied is not the same as in the present study, nor the constitutive model for the tailings material. The study conducted by Knutsson et al. (2015) contains deformation analyses and estimations of excess pore water pressures between the years 2014 and IGS 727 NGM Proceedings

2 MILLING FACILITY Modelling, analysis and design CLARIFICATION POND DECANT POND AITIK TAILINGS IMPOUNDMENT Figure 1 The studied tailings dam E-F is located between the tailings impoundment and the clarification pond. The red line shows the examined cross section of the tailings dam. (Knutsson, 2014) The results were used to assess the dam stability. By utilizing the factors of safety, FoS, alert values were estimated for monitoring programs. Studies regarding tailings material in the Aitik impoundment were carried out by Bjelkevik (2005) and Bhanbhro (2014) where behaviors and material properties of the tailings were investigated. 2 AITIK TAILINGS DAM The tailings dam studied is located in Aitik, Northern Sweden. The Aitik mine is an open pit copper mine that has been operational since 1968 and is owned by Boliden Mineral AB. The ore contains less than one percent of copper and with an annual production of 39 million tonnes of ore (year 2014) huge amounts of waste material are generated. The impoundment area, where the tailings material is deposited, is approximately 13 km 2 and is surrounded by tailings dams and by natural heights. The tailings dam E-F is chosen for the numerical analysis in this study and is located between the tailings impoundment and the clarification pond, see Figure 1. The most critical failure scenario occurs when a failure of tailings dam E-F results in a failure of the embankment dam surrounding the clarification pond. A failure like this might cause environmental damages downstream of the impoundment facilities. (Sweco Infrastructure AB and TCS AB, 2012) The tailings material is mixed with water from the processing plant to produce a slurry. Thereafter, the slurry is distributed through pipes and deposited in the impoundment. Between approximately the period 15 th of October and 30 th of April the tailings material is deposited from a single discharge point from tailings dam A-B, close to the milling facility, see Figure 1 (Sweco Infrastructure AB and TCS AB, 2012). This method is used to prevent freezing in the pipes. Between approximately the period 1 st of May and 14 th of October the spigot method (Blight, 2010) is used and the tailings are distributed through spigots placed around the impoundment, see Figure 1. The spigot method creates segregation in the tailings material; coarser particles are settled close to the dam structure and finer particles further away. This phenomenon with coarser and finer particles separated, creates good foundation conditions for future dike constructions. The coarser particles contribute to a denser foundation which is required by the upstream method. The dissipation of excess pore water pressures corresponds to the consolidation process where coarser grained soil have higher hydraulic conductivity and allow the phreatic line to disperse faster than finer grained soil. The area between the dam structure and the decant pond is called beach and is required to NGM Proceedings 728 IGS

3 Numerical analysis of an upstream tailings dam Note: Zones A to J represent tailings with different material properties. Figure 2 Material zones in the tailings dam. The red lines show the size of the dam year 2015 and the blue lines show the outer boundaries. The whole geometry is presented for year make the phreatic level decrease before reaching the downstream slope of the dam. (Lottermoser, 2010) The beach at tailings dam E-F is between 100 and 200 meters wide. (Sweco Infrastructure AB and TCS AB, 2012) 3 FINITE ELEMENT MODEL To increase impoundment storage for generated tailings, tailings dams have to be raised continuously with sufficient stability. According to Swedish recommendations, the stability of tailings dams has to be sufficient for a very long time; i.e years (Bjelkevik, 2005; Svensk Energi AB/SveMin, 2012). Svensk Energi AB/SveMin (2012) recommends factors of safety, FoS, for tailings dams to be above 1.5 during normal operation conditions. Plane strain conditions were applied in the finite element model since the dam was considered to be a long structure i.e. the length of the dam is much larger than the width of the dam; no deformations along the longitudinal direction were expected. By simulating future raisings at the tailings dam between the years 2024 and 2034 in the finite element software PLAXIS 2D, the dam stability was assessed. If the dam stability in the simulations results in FoS values below the recommended value, strengthening actions have to be implemented. In this numerical analysis, rockfill berms were placed on the downstream slope of the dam to increase the dam stability. Rockfill berms were assumed to be constructed on the downstream slope of the tailings dam between the period 15 th of October and 30 th of April, to increase the dam stability during the following dike construction. An optimization technique, where the volume of rockfill was minimized to fulfill the recommended stability, was utilized in the simulations. The basic idea with the technique was to place as small volumes of rockfill as possible on the downstream slope of the dam to obtain FoS values above the recommended value. The optimization technique used in this study was basically the same technique as used by Knutsson et al. (2015) and Ormann et al. (2013). In this paper and the paper by Knutsson et al. (2015) smaller rockfill berms were placed on several different places on the downstream slope, whilst Ormann et al. (2013) placed the whole volume of rockfill at only one additional suitable position on the downstream slope of the dam. The most suitable positions for the rockfill berms were found by trying various locations and different volumes. The procedure was repeated until a FoS above 1.5 was obtained for a reasonable small total volume of rockfill. 3.1 Geometry The geometry of the dam cross section was provided by the dam owner Boliden Mineral AB. The geometry was extended with future dike raisings with the same layout as the dikes constructed today (year 2015), see Figure 2. The width of the geometry was chosen sufficiently large to obtain realistic numerical results. The geometry was assumed to be closed for water flow in the left and the right IGS 729 NGM Proceedings

4 Modelling, analysis and design Table 1 Values of material properties for tailings in the dam structure in Figure 2 (Knutsson, 2014; Pousette, 2007; Bhanbhro, 2014). The material properties are defined in e.g. Brinkgreve et al. (2014). Unit A B C D E F G H I J ref E 50 ref E oed kpa kpa ref E ur kpa m p ref kpa ν ur c ref kpa φ ψ R f γ unsat kn/m γ sat kn/m e init k x m/days k y m/days 8.64E E E E E E vertical outer boundaries of the model, see Figure 2. The lower horizontal outer boundary was also assumed to be closed. Both horizontal and vertical deformations were modeled to not occur in the bottom boundary and for the left and right outer boundaries only vertical deformations were allowed. The foundation for tailings dam E-F as well as the starter dike consist of till, see Figure 2. The first six dike raisings, counted from the starter dike, were also constructed of till. The subsequent dike raisings were constructed of compacted tailings. All dike constructions were equipped with filters and rockfill on the downstream slope to protect the slope from surface erosion. The tailings were disposed and depending on the milling process of the tailings, the material properties vary over the years. The rockfill berms were constructed during the winter, due to less work activity at the impoundment that time compared to summer periods. Furthermore, rockfill materials are less sensitive to freezing and thawing than materials with lower hydraulic conductivity. 3.2 Material properties The geometry of the tailings dam studied was divided into different subareas to represent different soil layers, see Figure 2. The subareas have various sizes and different material properties. The values of the material parameters in Table 1 and Table 2 correspond to the letters and names shown in Figure 2. All values of the material properties for the tailings in the impoundment came from laboratory tests presented by Pousette (2007). The constitutive model Hardening Soil (Brinkgreve et al., 2014) was chosen for the tailings material, since parameter values were available. Values of material properties for the constitutive model were evaluated by Pousette (2007), Knutsson (2014) and Bhanbhro (2014). The values of material properties for the tailings evaluated for Hardening Soil can be seen in Table 1. By using Hardening Soil, in the analyses, deformations could be realistically modeled, since Hardening Soil is a better constitutive model than the simpler constitutive model NGM Proceedings 730 IGS

5 Numerical analysis of an upstream tailings dam Table 2 Values of material properties for filter, rockfill and till in the dam structure in Figure 2 (Sweco Infrastructure AB and TCS AB, 2012; Jonasson, 2008). The material properties are defined in e.g. Brinkgreve et al. (2014). Unit Filter Rockfill Till (starter dike) Till (underground) E kpa v c ref kpa φ ψ γ unsat kn/m γ sat kn/m e init k x m/days k y m/days Mohr Coulomb when analyzing deformations (Brinkgreve et al., 2014). The constitutive model Mohr Coulomb was chosen for filter, rockfill and till since values of material properties were available, see Table 2. The values of material properties for the Mohr Coulomb model were provided from geotechnical investigations reported by Jonasson (2008). All values used as material properties in the simulations were assumed to be valid also in the future. No studies about future behavior of this tailings material have been composed; for instance, regarding changes of stiffness and strength caused by aging effects. The increased load from the dike constructions is first carried by the pore water in the form of excess pore water pressures. The excess pore water pressures will then slowly decrease during consolidation and the load transforms gradually over to the soil skeleton. The hydraulic conductivity in the soil material and drainage conditions in the surroundings determines the rate of excess pore water pressure dissipation. This is modeled in PLAXIS 2D for each activity, by first assuming undrained conditions that will change into drained conditions if complete consolidation is achieved. 3.3 Mesh generation The geometry of the tailings dam was divided into 15-noded triangular finite elements. In order to choose a proper mesh, different element coarseness were tested in the simulations. The most convenient mesh was chosen considering both computation time and accuracy of the results, see Figure 3. Local mesh refinements were performed in the downstream toe of the dam to achieve more accurate results in the computations. Figure 3 Generated mesh in the model with local mesh refinements at the dam toe. 3.4 Computation phases All computations in the software PLAXIS 2D were defined with initial conditions, described in an initial phase at a pre-defined time. This initial time point for the studied tailings dam E-F was chosen to be in year 2007, since the raising rate of the dam increased that year. It was assumed, due to the low raising rate earlier, that no excess pore water pressures existed in the dam structure at the defined initial time point. All IGS 731 NGM Proceedings

6 Modelling, analysis and design deformations were set to zero in the initial phase. One year was divided into different activities that were inserted in the simulations as computation phases, see Figure 4. Between the period 15 th of October and the 30 th of April, no discharge of tailings from dam E-F was taking place. This period is called winter in Figure 4. In the vicinity of the analyzed cross section, it was assumed that spigotting of tailings took place between the period 1 st of May and 31 st of July. Followed by a period of 15 days of rest, 15 days of dike construction and another 15 days of rest. The rests were needed for the working process for dike constructions. After that, spigotting continued 14 th of September until 14 th of October. Month January February March April May June July August September October November December Activities (120 days) (121 days if leap year) Spigotting (91 days) Rest (15 days) Dike construction (15 days) Rest (15 days) Spigotting (31 days) (78 days) Figure 4 Annual activities at dam E-F used in the computations. In order to assess the slope stability of tailings dam E-F, consolidation computations were executed. The times defined in Figure 4 correspond to the duration of each activity where consolidation was computed. The worst case was found when the load increased within the dam structure and consolidation was still not completed. This occurred immediately after a new dike was constructed. The increased load gave increased excess pore water pressures and thus decreased FoS values. With the intention of examining the dam stability, safety computations were performed after each consolidation computation. The factor of safety is defined in PLAXIS 2D by FoS = available strength strength at failure (1) where the available strength is the strength from the effective stress state and input strength parameters. The strength at failure is obtained by decreasing the input strength parameters in small increments until an unstable structure is obtained. The FoS assessed in PLAXIS 2D gives the global FoS of the whole dam geometry. (Brinkgreve et al., 2014) 3.5 Water conditions The phreatic line defined in the simulations was assumed to be at the surface of the tailings material, see a), b) and c) in Figure 5 depending on the activity described in Figure 4. As can be seen, the location of the phreatic line was changing depending on the activity. When the phreatic line reached the point where the beach and the dike construction intersects (dike toe) the groundwater level started to incline as a straight line between fixed measurement points in the dikes, see d) in Figure 5. Thereafter, the phreatic line was assumed to be horizontal. This level represents the highest allowed water level in the clarification pond. The field measurements regarding ground water location were provided by Boliden Mineral AB. 4 RESULTS Stability analyses were conducted to assess the future stability of the studied tailings dam. The stability of the tailings dam, subjected to raisings, was first analyzed without rockfill berms added on the downstream slope of the dam. The result indicated a trend of decreasing FoS. Strengthening actions were therefore required. This was modeled by adding rockfill berms on the downstream slope. NGM Proceedings 732 IGS

7 Factor of safety Numerical analysis of an upstream tailings dam a) b) c) d) Figure 5 Location of the phreatic line: a) between 14 th of September and 30 th of April b) between 1 st of May and 14 th of August c) between 15 th of August and 13 th of September d) in the dam toe during all activities. 4.1 Stability with no strengthening methods The studied raisings of the tailings dam were simulated without any strengthening methods on the downstream slope of the dam to investigate if sufficient stability was obtained. It was shown in the simulations that after only three years the FoS was decreasing to values continuously below the recommended value of 1.5. Since the FoS did not fulfill the recommended value no further analyses without strengthening methods were performed. The result from these simulations gave the FoS shown in Figure 6. 1,7 1,6 1,5 1, , Year Figure 6 Values of FoS with no rockfill berms added on the downstream slope of the dam. The highest computed FoS value each year occurs when maximum consolidation is achieved, due to decreased excess pore water pressures in the dam structure, see a) in Figure 7. Maximum consolidation is obtained immediately before spigotting in the spring i.e. 30 th of April. When the spigotting begins, 1 st of May, the FoS starts to decrease. This phenomenon occurs because of increased load from newly deposited tailings. The increased load contributes to excess pore water pressures and thereby reduced stability. Later in August when a new dike construction takes place, the FoS makes a rapid drop due to increased load which contributes to excess pore water pressures in the dam, see b) in Figure 7. The lowest FoS corresponds to the time when maximum excess pore water pressures are obtained, see c) in Figure 7. When consolidation of the tailings material continues after a dike is constructed, the FoS starts to increase again. a) Maximum consolidation b) New dike construction c) Consolidation starts again Figure 7 Enlargement of the red rectangle in figure 6. a) maximum consolidation in the soil is obtained, b) load from a new dike construction is applied, c) the soil begins to consolidate, highest values of excess pore water pressure are also find in this phase. The most critical slip surface in year 2027, associated with the lowest FoS, occurred directly after a new dike was constructed, see Figure 8. The slip surface is located in the upper part of the downstream slope of the dam and the shape is almost circular. Since the FoS is below the recommended value of 1.5, the dam structure was further investigated by adding rockfill berms on the downstream slope of the dam. 4.2 Stability with strengthening methods The FoS from the simulations with rockfill berms added on the downstream slope of the dam can be seen in Figure 9. The figure shows that the FoS for the whole time period studied are above the recommended value of 1.5. It can also be noted in Figure 9 that the IGS 733 NGM Proceedings

8 Factor of safety Modelling, analysis and design annual distribution of the values of FoS is very similar each year. The highest value of the FoS occurred when maximum consolidation was achieved and the lowest value of the FoS was achieved immediately after a new dike construction, due to increased load giving increased excess pore water pressures. The positions of the added rockfill berms can be seen in Figure 10. The rockfill berms added on the downstream slope of the dam increased the FoS so that values above the recommended value were obtained in all computation phases. The slip surface representing the lowest FoS in year 2027 with rockfill berms added on the downstream slope of the dam can be seen in Figure 11. The slip surface is located deep, throughout the whole dam body and embraces a much larger volume of soil than the slip surface in Figure 8. The slip surface obtained in Figure 11 is non-circular. The volumes of rockfill modeled in the simulations, during winters, on the downstream slope of the dam can be seen in Figure 12. The volumes were calculated with a constant dam length of 1500 meters. Thus, the south end of the dam consists of another dam and the north end of dam E-F consists of a dam corner, see Figure 1. It was noted that different volumes of rockfill were needed different years to obtain values of the FoS above the recommended value. Between the years 2030 and 2033, larger amounts of rockfill were needed compared to the other years studied. 5 CONCLUDING REMARKS It was shown that if no rockfill berms were added on the downstream slope of the dam the slip surfaces were almost circular and located in the upper part of the dam structure. However, the factor of safety, FoS, gradually decreased below the recommended value of 1.5 as the dam was raised. Strengthening actions were required to obtain values of the FoS above the recommended value. Rockfill berms were thus added in the simulations on the downstream slope of the dam structure. It was stated that with enough rockfill berms on the downstream slope of the dam the stability between the years 2024 and 2034 was sufficient according to the recommended value of 1.5. When rockfill berms were added on the downstream slope of the dam the values of the FoS increased and the shape of the slip surfaces changed, they started to go deep through the whole dam and embraced larger volumes of soil. The distribution of the FoS has a similar trend during each year in the analyses with rockfill berms. The FoS are located above the recommended value in all simulations which FoS = 1.39 Figure 8 The most critical slip surface after dike construction year 2027; without any rockfill berms added. 1,7 1,6 1,5 1,4 1, Year Figure 9 FoS when rockfill berms were added on the downstream slope of the dam. NGM Proceedings 734 IGS

9 Volume [Mm 3 ] Numerical analysis of an upstream tailings dam Figure 10 Rockfill plan for future raisings at the studied dam E-F. FoS = 1.50 Figure 11 The most critical slip surface after dike construction year 2027; with rockfill berms added. 1,4 1,2 1 0,8 0,6 0,4 0, Year Figure 12 Volume rockfill used on the downstream slope of the dam in the simulations indicated that a good optimization of rockfill berms was performed. Different volumes of rockfill had to be added each year in order to increase the values of FoS. The increased dam stability comes from the resisting moment of the added berms and if longer distances, i.e. levers, between the point of rotation and the point of gravity in the berms were utilized, less amount of rockfill had to be used. Once a geometry of a structure was inserted into a finite element software it is easy to do further investigations and analyses, by stepwise add or remove soil volumes to the structure. Thus, gradual raisings of a tailings dam is a good example of an engineering application that is particularly suitable to analyze in a finite element software. This study has shown, that the finite element software PLAXIS 2D is a very good tool in finding the most convenient locations as well as volumes for rockfill berms to be added, using stability analysis when raising tailings dams. 6 ACKNOWLEDGEMENTS The authors would like to express their sincere thanks to Boliden Mineral AB, Sweden for giving us this opportunity to carry out the presented study regarding the tailings dam E-F at the Aitik mine. The authors would also like to express special thanks to Mr. Peter Marthin and Mr. Anders Forsgren, Boliden Mineral AB and Ms. Annika Bjelkevik, TCS AB for encouraging support and site information. IGS 735 NGM Proceedings

10 Modelling, analysis and design Mr. Riaz Bhanbhro at Luleå University of Technology (LTU), is to be acknowledged for performing laboratory tests and for evaluation of material parameters and Mr. Fredrik Jonasson at SWECO Energuide AB, for providing information from field data. The research presented was carried out within the environment of "Swedish Hydropower Centre - SVC" at LTU. The support from the SVC environment is highly appreciated and acknowledged for. SVC was established by the Swedish Energy Agency, Elforsk and Svenska Kraftnät together with Luleå University of Technology, The Royal Institute of Technology, Chalmers University of Technology and Uppsala University. Luleå University of Technology is acknowledged for additional financial support and for providing laboratory resources for the work. Ormann, L., Zardari, M.A., Mattsson, H., Bjelkevik, A. & Knutsson, S. (2013) Numerical analysis of strengthening by rockfill embankments on an upstream tailings dam. Canadian Geotechnical Journal, 50, Pousette, K. (2007) Laboratorieförsök på anrikningssand från Aitik. Ödometerförsök, Triaxialförsök. Luleå, Sweden: Luleå University of Technology. (In Swedish) (Internal working document) Svensk Energi AB/SveMin. (2012) GruvRIDAS - Gruvindustrins riktlinjer för dammsäkerhet. Stockholm, Sweden: Föreningen för gruvor, mineraloch metallproducenter i Sverige. (In Swedish) Sweco Infrastructure AB, TCS AB. (2012) Bilaga A5 - Teknisk Beskrivning Dammar - Förutsättningar för nuvarande och fortsatt deponering i befintligt sandmagasin vid Aitikgruvan. Stockholm, Sweden: Sweco Infrastructure AB, TCS AB. Project number: and (In Swedish) (Internal report) Vick, S.G. (1990) Planning, Design, and Analysis of Tailings Dams. Vancouver, Canada: Bitech Publishers Ltd. 7 REFERENCES Bhanbhro, R. (2014) Mechanical Properties of Tailings Basic Description of a Tailings Material from Sweden. Licentiate thesis. Luleå, Sweden: Graphic Production. Bjelkevik, A. (2005) Water Cover Closure Design for Tailings Dams State of Art Report. Licentiate thesis. Luleå, Sweden: Universitetstryckeriet. Blight, G. (2010) Geotechnical Engineering for Mine Waste Storage Facilities. London, U.K.: Taylor & Francis Group. Brinkgreve, R.B.J., Engin, E. & Swolfs, W.M. (2014) Plaxis Delft, the Nederlands: Plaxis bv. Jonasson, F. (2008) PM Förslag på materialparametrar för Övriga material vid beräkning i PLAXIS. Project number: , Luleå, Sweden: SWECO VBB (In Swedish) (Internal report) Knutsson, R. (2014) Numerical analyses of Aitik tailings dam. Luleå, Sweden: Luleå University of Technology. (Internal working document) Knutsson, R., Viklander, P. & Knutsson, S. (2015) The use of numerical modeling in alert level set-up for instrumentation in tailings dams. In Proceedings of 25 th Congress and 83 rd Annual Meeting of International Commission of Large Dams (ICOLD), Stavanger, Norway, June Q.98-R. Lottermoser, B. (2010) Mine wastes - Characterization, Treatment and Environmental impacts. New York, U.S.: Springer. Ormann, L., Zardari, M.A. Mattsson, H., Bjelkevik, A. & Knutsson, S. (2011) Numerical Analysis of Curved Embankment of an Upstream Tailings Dam. Electronical Journal of Geotechnical Engineering (EJGE), 16, NGM Proceedings 736 IGS